Frequency synthesis with reference signal generated by opportunistic phase locked loop

ABSTRACT

Systems, methods, and circuitries are provided to generate a radio frequency (RF) signal having a desired radio frequency f RF . In one example a frequency synthesizer system includes a clock, an opportunistic phase locked loop (PLL), and an RF PLL. The clock circuitry is configured to generate a clock signal having a frequency f XTL . The opportunistic phase locked loop (PLL) is configured to generate a reference signal having a reference frequency f REF  that is close to a free-running frequency of an oscillator in the opportunistic PLL. The opportunistic PLL is configured to synchronize the reference signal to the clock signal. The RF PLL is configured to generate the RF signal having the desired radio frequency and to synchronize the RF signal with the reference signal.

BACKGROUND

Phase locked loops (PLLs) are often used to generate local oscillator(LO) signals at arbitrary frequencies for converting signals between thebaseband and RF domains in a device. A PLL synchronizes the RF signalbeing generated by an oscillator circuit with a reference signal toensure that that the RF signal is phase-aligned with respect to othersignals in the device.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 illustrates an exemplary frequency synthesizer system that usescrystal-based clock circuitry to generate a reference signal for an RFPLL.

FIG. 2 illustrates an exemplary frequency synthesizer system thatincludes an opportunistic PLL generating a reference signal for an RFPLL in accordance with various aspects described.

FIG. 3 illustrates an exemplary flow diagram of an exemplary method ofgenerating an RF signal having a desired RF frequency in accordance withvarious aspects described.

FIG. 4 illustrates an exemplary flow diagram of another exemplary methodof generating an RF signal having a desired RF frequency in accordancewith various aspects described.

FIG. 5 illustrates an exemplary opportunistic PLL frequency adjustmentrange mapped to an exemplary RF frequency band in accordance withvarious aspects described.

FIG. 6 illustrates an exemplary RF frequency band mapped to an exemplaryopportunistic PLL frequency adjustment range for eight different RF PLLratio sub-integer portions in accordance with various aspects described.

FIG. 7 illustrates an exemplary frequency synthesizer system thatincludes an opportunistic PLL generating a reference signal for two RFPLLs in accordance with various aspects described.

FIG. 8 illustrates an exemplary RF circuitry in accordance with variousaspects described.

DESCRIPTION

Radio frequency (RF) communication systems rely on generating RF signalshaving a wide range of frequencies. As throughput of devices increasesso does the demand for the spectral purity of the RF signals. The nextgeneration of radio transceiver standards (such as advanced LTE, 5Gmillimeter-wave and 802.11ax Wi-Fi) impose extremely high requirementson the spectral purity of local frequency oscillators (LO) while at thesame time demanding dense channel spacing in a broad spectral range.

All digital phase locked loops (ADPLLs) are commonly used for localfrequency generation in communication systems. These systems use a fixedfrequency reference standard (often referred to as a REF clock) which isusually based on a highly stable quartz or crystal resonator. FIG. 1illustrates a typical ADPLL based frequency synthesizer system 10 thatutilizes a fixed reference signal generated by a crystal based clockcircuitry 15 to generate an RF signal having frequency f_(RF). An RF PLL40 locks the phase of a high-frequency oscillator 46 to the clock signal(having relatively low frequency f_(XTL)). While the clock circuitry 15includes a crystal, any other clock circuitry that is used tosynchronize the operation of components across multiple circuits may beused to implement the clock circuitry 15. The term PLL as used hereinincludes ADPLLs as well as any other PLL and should not be limited torefer only to ADPLLs.

A time-to-digital converter (TDC) 42 quantifies a phase differencebetween the reference signal and the RF signal output by the RF PLL 40.Out of this digital difference, the phase error is computed. The TDC 42is operated to quantify the phase difference between an edge of theclock signal and every Nth edge of the RF signal, which is equivalent to“dividing” the feedback RF signal frequency by N. The quantity N (calledthe ratio herein), which can be an integer or an integer portioncombined with a sub-integer portion, is used by the PLL to control therelationship between f_(RF) and f_(REF).

The phase error output by the TDC 42 is filtered by a digital loopfilter 44 and the filtered signal is used to adjust the high-frequencydigitally controlled oscillator (DCO) 46 to bring the RF signal intophase with the clock signal. The operation of the RF PLL 40 can beviewed as a filtered frequency multiplication of reference signal (byratio N) to generate the RF signal. While in this description themultiplication/division by the ratio is conceptually illustrated asbeing a function of the TDC, the systems, methods, and circuitriesdescribed herein are also applicable to PLLs in which differentcircuitry performs this operation.

Depending on the fractional frequency generation technique employed(either divider-less or sigma-delta driven multiple modulus divider(MMD) based). The periodic phase errors can manifest themselves in theform of coherent spurious signals or an elevated in-band noise floor. Inthe case of a divider-less ADPLL periodic phase errors acquire spuriousmanifestation and are called fractional spurs. The phase noiseperformance of an ADPLL is dominated by the clock signal phase noisebehavior and the periodic phase errors generated in the ADPLL due to theratio N (f_(RF)/f_(REF)). To improve the ADPLL phase noise performancehigh frequency and therefore prohibitively expensive crystals are used.Hence, there is an inherent limit on achievable RF signal spectralpurity.

Main drawbacks of using a fixed reference frequency as in system 10include performance limitations and periodic errors. Reference signalnoise scales according to 20 log₁₀(f_(RF)/f_(REF)) (recall thatf_(RF)/f_(REF)=N). Thus, performance is degraded as the ratio betweenthe desired RF frequency and the fixed reference signal frequencyincreases. This means that performance is limited to the rangesproducible by crystal-based oscillators. Further, significant periodicerrors occur due to the nature of the fractional PLL which limitsusability in dense channelization standards, such as 5G.

Described herein are systems, circuitries, and methods that produce anRF signal with a high spectral purity by using an opportunistic PLL togenerate a reference signal of intermediate frequency that issynchronized with a low frequency oscillator such as a crystaloscillator. The opportunistic PLL includes a high-performance narrowbandwidth DCO that generates a reference signal having an arbitrary, butmodestly tunable, frequency which helps to alleviate performanceimpairments common to fractional frequency synthesis and provides a widechoice of effective ratios in each of the locked loops (opportunisticand RF). In this manner a very low jitter PLL is created which is lockedto an arbitrary frequency (called herein “opportunistic”) using a narrowbandwidth digitally controlled oscillator (DCO) (featuring low parasiticcontent due having limited tuning mechanisms). This high-performance DCOenables the opportunistic PLL to apply a narrow phase-locking bandwidth,which in turn contributes significant filtering of fractional spurs andalleviating other problems caused by use of reference signals having afixed relatively low frequency. The systems, circuitries, and methodsdescribed herein handle and mitigate in-band noise and spurious signalsas well as introducing a better trade-off for high-performance PLLs.

The present disclosure will now be described with reference to theattached figures, wherein like reference numerals are used to refer tolike elements throughout, and wherein the illustrated structures anddevices are not necessarily drawn to scale. As utilized herein, terms“module”, “component,” “system,” “circuit,” “element,” “slice,”“circuitry,” and the like are intended to refer to a set of one or moreelectronic components, a computer-related entity, hardware, software(e.g., in execution), and/or firmware. For example, circuitry or asimilar term can be a processor, a process running on a processor, acontroller, an object, an executable program, a storage device, and/or acomputer with a processing device. By way of illustration, anapplication running on a server and the server can also be circuitry.One or more circuits can reside within the same circuitry, and circuitrycan be localized on one computer and/or distributed between two or morecomputers. A set of elements or a set of other circuits can be describedherein, in which the term “set” can be interpreted as “one or more.”

As another example, circuitry or similar term can be an apparatus withspecific functionality provided by mechanical parts operated by electricor electronic circuitry, in which the electric or electronic circuitrycan be operated by a software application or a firmware applicationexecuted by one or more processors. The one or more processors can beinternal or external to the apparatus and can execute at least a part ofthe software or firmware application. As yet another example, circuitrycan be an apparatus that provides specific functionality throughelectronic components without mechanical parts; the electroniccomponents can include one or more processors therein to executeexecutable instructions stored in computer readable storage mediumand/or firmware that confer(s), at least in part, the functionality ofthe electronic components.

It will be understood that when an element is referred to as being“connected” or “coupled” to another element, it can be physicallyconnected or coupled to the other element such that current and/orelectromagnetic radiation (e.g., a signal) can flow along a conductivepath formed by the elements. Intervening conductive, inductive, orcapacitive elements may be present between the element and the otherelement when the elements are described as being coupled or connected toone another. Further, when coupled or connected to one another, oneelement may be capable of inducing a voltage or current flow orpropagation of an electro-magnetic wave in the other element withoutphysical contact or intervening components. Further, when a voltage,current, or signal is referred to as being “applied” to an element, thevoltage, current, or signal may be conducted to the element by way of aphysical connection or by way of capacitive, electro-magnetic, orinductive coupling that does not involve a physical connection.

As used herein, a signal that is “indicative of” a value or otherinformation may be a digital or analog signal that encodes or otherwisecommunicates the value or other information in a manner that can bedecoded by and/or cause a responsive action in a component receiving thesignal. The signal may be stored or buffered in computer readablestorage medium prior to its receipt by the receiving component and thereceiving component may retrieve the signal from the storage medium.Further, a “value” that is “indicative of” some quantity, state, orparameter may be physically embodied as a digital signal, an analogsignal, or stored bits that encode or otherwise communicate the value.

Use of the word example is intended to present concepts in a concretefashion. The terminology used herein is for the purpose of describingparticular examples only and is not intended to be limiting of examples.As used herein, the singular forms “a,” “an” and “the” are intended toinclude the plural forms as well, unless the context clearly indicatesotherwise. It will be further understood that the terms “comprises,”“comprising,” “includes” and/or “including,” when used herein, specifythe presence of stated features, integers, steps, operations, elementsand/or components, but do not preclude the presence or addition of oneor more other features, integers, steps, operations, elements,components and/or groups thereof.

In the following description, a plurality of details is set forth toprovide a more thorough explanation of the embodiments of the presentdisclosure. However, it will be apparent to one skilled in the art thatembodiments of the present disclosure may be practiced without thesespecific details. In other instances, well-known structures and devicesare shown in block diagram form rather than in detail in order to avoidobscuring embodiments of the present disclosure. In addition, featuresof the different embodiments described hereinafter may be combined witheach other, unless specifically noted otherwise.

FIG. 2 illustrates an exemplary frequency synthesizer system 100 thatincludes an opportunistic PLL 120 to generate a reference signal for anRF PLL 140. The frequency of the reference signal generated by theopportunistic PLL 120 is about two orders of magnitude greater thanfrequencies that can be produced by a crystal based clock circuitry 115while preserving or even improving the phase noise quality. The clocksignal produced by the clock circuitry 115, which has a frequencyf_(XTL), is used to lock the long-term phase drift of a high-performanceoscillator 126 (in one example the oscillator 126 is a DCO) withoutcompensating for the center frequency shift of the oscillator 126 due topower/voltage/temperature (PVT) changes. The oscillator 126 iscontrolled to generate a reference signal having frequency f_(REF) thatis chosen to coincide with an optimum oscillation frequency dictated bythe process where the figure of merit (FOM) is optimized. The oscillator126 has a modest tuning range (less than 1% or on the order of f_(XTL))which means that many noise-producing tuning components such as switchedcapacitor banks can be eliminated, greatly enhancing the stability ofthe reference signal generated by the opportunistic PLL 120.

For the purposes of this description, the term “opportunistic PLL”includes low-power, narrow bandwidth oscillators that are locked to alow frequency crystal-based clock circuitry (or other low frequencysystem clock source). To generate the reference signal, theopportunistic PLL is first operated in an open-loop mode in which theoscillator (or DCO) in the opportunistic PLL operates at a frequency ator near its free-running (or uncompensated or natural) frequency. Theopportunistic PLL is then operated in closed loop mode so that thereference signal is synchronized with the clock signal. Once locked intophase with the reference clock, the phase of the signal produced by theopportunistic PLL does not vary appreciably over time and can beconsidered to be deterministic while the frequency will have a valuethat is dependent on the operating conditions of the oscillator when theloop is locked. The oscillator in the opportunistic PLL is capable oftuning only within a narrow range around its free-running frequency.Thus the reference signal produced by the opportunistic PLL 120 willhave a high spectral purity and an operating frequency that is mainlydetermined by the optimum oscillator frequency for given manufacturingprocesses with some limited tunability. By employing a narrow bandwidthloop, both the clock circuitry and TDC contributed noise and errors aresignificantly suppressed, which allows for use of a low cost crystal inthe clock circuitry 115 and a low power TDC.

The opportunistic PLL 120, when used as a source for the referencesignal for the RF PLL 140, provides two advantages over directly usingthe clock signal as the reference signal for the RF PLL 140. Theseadvantages are the increased frequency of the reference signal and theability to make (minor) adjustments to the reference frequency. Incommon design practice (FIG. 1) the noise contribution of the crystaloscillator in the clock circuitry can be calculated via

$\begin{matrix}{{PN}_{XO} + {20\log_{10}\frac{F_{RF}}{F_{REF}}}} & {{EQ}.\mspace{14mu} 1}\end{matrix}$

Thus, for instance, a 60 MHz crystal oscillator phase noise thatflattens at 1 MHz offset to a noise floor of −160 dBc/Hz, when used as aclock circuitry for a PLL that generates an RF signal at 6 GHz willcontribute an equivalent noise of

$\begin{matrix}{{{{- 1}60\frac{dBc}{Hz}} + {20\log_{10}\frac{6\mspace{14mu}{GHz}}{60\mspace{14mu}{MHz}}}} = {{- 1}20\frac{dBc}{Hz}}} & {{EQ}.\mspace{14mu} 2}\end{matrix}$When a high performance 3 GHz DCO, featuring a phase noise of

${- 1}35\frac{dBc}{Hz}$at 1 MHz offset, is used to generate the reference signal, it will havecontributed to the same 6 GHz signal significantly less phase noise

$\begin{matrix}{{{{- 1}35\frac{dBc}{Hz}} + {20\log_{10}\frac{6\mspace{14mu}{GHz}}{3\mspace{14mu}{GHz}}}} = {{- 1}29\frac{dBc}{Hz}}} & {{EQ}.\mspace{14mu} 3}\end{matrix}$

It can be seen that by using a high frequency (as compared to the clocksignal frequency) reference signal the phase noise is reduced by 9 dB.Furthermore, by using a higher frequency reference signal, quantizationerror of the TDC 122 is spread over larger bandwidth, and hence itsdensity decreases by the ratio between reference signal's high frequencyand the XTAL frequency.

Having control over the reference frequency (by tuning the oscillator126) provides the ability to operate the RF PLL 140 using a ratio N thatis selected to generate an RF signal in which spurs are located far outof band and/or are highly rejected by the digital loop filter (DLF) 144.A fractional spur is a periodic error created by the phase wrapping inthe TDC 142 (in a first order sigma-delta divider-less DPLL). When theratio is close (but not equal) to either an integer or sub-integer(1-over-small-number), the error has a period that is close to the RFsignal frequency. This makes it extremely difficult to filter out theerror due to the low-pass nature of the clock circuitry transferfunction. The variable reference signal frequency allows the frequencysynthesizer system 100 to avoid periodic phase errors in the RF PLL 140by selecting the ratios used in the opportunistic PLL 120 and the RF PLL140 so that any resulting periodic error is filtered out by at least oneof the closed loop transfer functions.

Recall that the frequency of the reference signal is not set a priori,but rather is an optimum (e.g., free-running) frequency that is lockedinto phase with the clock signal. This means that the reference signalfrequency should be measured. Control circuitry 160 includes measurementcircuitry 165 configured to measure the frequency of the referencesignal. Based on this measured reference frequency and the desired RFfrequency, the control circuitry determines a “good” ratio N that tendsto avoid periodic phase errors. The control circuitry 160 then controlsor adjusts the oscillator 126 to compensate for the selected ratio Nsuch that the reference signal, when multiplied by the RF PLL ratio,will generate the desired RF frequency. In one example, the measurementcircuitry 165 includes a simple cycle counter. The measurement circuitry165 may also combine the measurement of the counter with informationgathered by the TDC 122.

Because the reference frequency is determined based on the free-runningfrequency of the oscillator in the opportunistic PLL and the oscillatorhas a very limited tuning capability, as operating conditions change a“new” reference frequency may be selected that reflects thecharacteristics of the oscillator in the new operating conditions. thecontrol circuitry 160 is configured to monitor certain “re-calibration”criteria such as temperature, supply power, elapsed time since a lastcalibration, or even a change in the desired RF signal frequency. Inresponse to a re-calibration criterion being met, the control circuitry160 controls the opportunistic PLL to generate a new reference signal byoperating in an open loop condition such that the oscillator 126operates at a new free-running frequency for the oscillator and then thecontrol circuitry controls the opportunistic PLL to operate in a closedloop condition (“locking the loop”) such that a new f_(REF) isapproximately the new free-running frequency and the new referencesignal is synchronized with the clock signal. The control circuitry 160then controls the RF PLL 140 to determine a new ratio (N) between a newmeasured f_(REF) and the RF frequency; adjust the new f_(REF) based onthe ratio; and control the DCO 146 based on a phase difference betweenthe new reference signal and the feedback RF signal to synchronize theRF signal with the new reference signal.

FIG. 3 illustrates a flow diagram of an exemplary method 300 to generatea desired radio frequency (RF) signal having a desired RF frequencyf_(RF). The method 300 may be performed, for example, by the frequencysynthesizer system 100 of FIG. 2. The method includes, at 310, with aclock circuitry, generating a clock signal having a frequency f_(XTL).At 320, the method includes generating a reference signal having areference frequency f_(REF). At 330 the method includes synchronizingthe reference signal with the clock signal with an opportunistic phaselocked loop (PLL). At 340 the RF signal having the desired radiofrequency is generated. At 350, the method includes synchronizing the RFsignal with the reference signal using an RF PLL.

FIG. 4 illustrates a flow diagram of an exemplary method that may beperformed by control circuitry of the frequency synthesizer system 100of FIG. 2 to determine an integer portion and sub-integer portion of theratio N used by the TDC 142. The method includes, at 410, determiningthe desired f_(RF). At 420, the reference frequency f_(REF) is measureddirectly and/or using information from PLL components. At 430, aninteger portion (A) of the ratio (or band selection) is determined basedon f_(RF)/f_(REF). The method includes, at 440, determining a frequencymodulo Δf_(RF) based on f_(RF)−A·f_(REF) and, at 450, selecting asub-integer ratio denominator (B) (or resolution of the intra-integertuning) based on the frequency modulo. At 460 a closest sub-integerratio numerator (n) is determined based on B·Δf_(RF)/f_(REF). At 470 theratio (N) for RF PLL 140 is set to A+n/B. At 480 the opportunistic PLLis controlled to generate a reference signal having the referencefrequency f_(RF)/N. In other examples, different methods may be used todetermine or set the integer and sub-integer portions of the ratio basedon a measured reference frequency and a desired RF frequency.

FIG. 5 illustrates how a minimal opportunistic PLL tuning range ofaround 2500 parts per million (ppm) can cover a 250 MHz frequency bandgiven an RF PLL with a ratio sub-integer portion having 1/8 resolution.For example, if the desired RF frequency is 12,100 MHz then thesub-integer portion of the ratio N for the RF PLL 140 should be selectedas 3/8 and the oscillator 126 (FIG. 2) should be adjusted by +500 ppm.While FIG. 5 shows a minimal tuning range for the opportunistic PLL willcover a frequency band of 250 MHz, FIG. 6 illustrates that by having asomewhat wider tuning range of 5000 ppm provides additional flexibilityin selecting the RF PLL sub-integer portion of the ratio. For example,if the desired frequency is 12,100 MHz then the sub-integer portion canbe selected as 3/8 with +500 ppm adjustment or 2/8 with +3000 ppmadjustment. In this manner, if any of the possible sub-integer portionsof ratios would result in in-band spurs or harmonics, anothersub-integer portion can be selected (e.g., by control circuitry 160 ofFIG. 2) and the opportunistic PLL oscillator adjusted accordingly. Inone example, if one of the possible sub-integer portions is 0/8 (e.g.,the ratio is an integer), one of the other possible sub-integer portions(e.g., the ratio is a non-integer) could be chosen to avoid harmonics.

FIG. 7 illustrates an exemplary frequency synthesizer system 700 thatincludes the system 100 of FIG. 2 and an additional RF PLL 740 thatgenerates a second RF signal having a second RF frequency. The RF PLL740 uses the reference signal generated by the opportunistic PLL 120 asa reference signal. The RF PLL 740 will have a different ratio that isdetermined based on the second RF frequency and the present value ofreference signal frequency. In one example the RF PLL 740 generates anRF signal having the same frequency as the RF signal generated by the RFPLL 140 but at a lower power or quality. This allows flexibility inusing the low power RF PLL 740 when conditions allow. Thus theopportunistic PLL 120 can be used as a “high quality” reference signalsource to multiple other PLLs that are able to adapt their operationbased on the present value of the reference signal frequency.

It can be seen from the foregoing description that using anopportunistic PLL to generate a stable reference signal of relativelyhigh frequency while providing modest tuning capability yields a lowpower solution for generation of very low phase noise RF signals. Thisleads to improved error vector magnitude (EVM) of both the transmitterand receiver in high-performance demanding standards such as 5G and nextgeneration Wi-Fi protocols. Also stringent requirements for the crystal(frequency, accuracy, and so on) are substantially relaxed so that lowercost components can be used.

FIG. 8 illustrates RF circuitry 800 according to some aspects. In anaspect, RF circuitry 800 may include one or more of each of IF interfacecircuitry 805, filtering circuitry 810, upconversion and downconversioncircuitry 815, synthesizer circuitry 820, filtering and amplificationcircuitry 825, power combining and dividing circuitry 830 and radiochain circuitry 835. The upconversion and downconversion circuitry 815may include the frequency synthesizer systems 100, 200 of FIGS. 2 and 3and/or perform the methods 300, 400 of FIGS. 3 and 4.

While the invention has been illustrated and described with respect toone or more implementations, alterations and/or modifications may bemade to the illustrated examples without departing from the spirit andscope of the appended claims. In particular regard to the variousfunctions performed by the above described components or structures(assemblies, devices, circuits, systems, etc.), the terms (including areference to a “means”) used to describe such components are intended tocorrespond, unless otherwise indicated, to any component or structurewhich performs the specified function of the described component (e.g.,that is functionally equivalent), even though not structurallyequivalent to the disclosed structure which performs the function in theherein illustrated exemplary implementations of the invention.

Examples can include subject matter such as a method, means forperforming acts or blocks of the method, at least one machine-readablemedium including instructions that, when performed by a machine causethe machine to perform acts of the method or of an apparatus or systemfor using an opportunistic PLL to generate a reference signal andgenerating an RF signal synchronized to the reference signal accordingto embodiments and examples described herein.

Example 1 is a frequency synthesizer system configured to generate aradio frequency (RF) signal having a desired radio frequency f_(RF),including a clock circuitry, an opportunistic phase locked loop (PLL),and an RF PLL. The clock circuitry is configured to generate a clocksignal having a frequency f_(XTL). The opportunistic PLL is configuredto generate a reference signal having a reference frequency f_(REF) thatis close to a free-running frequency of an oscillator in theopportunistic PLL. The opportunistic PLL is configured to synchronizethe reference signal to the clock signal. The RF PLL configured togenerate the RF signal having the desired radio RF frequency f_(RF) andto synchronize the RF signal with the reference signal.

Example 2 includes the subject matter of example 1, including oromitting optional elements, wherein the reference frequency f_(REF) isat least one order of magnitude greater than f_(XTL).

Example 3 includes the subject matter of example 1, including oromitting optional elements, wherein the opportunistic PLL is configuredto generate the reference signal in an open loop condition wherein theoscillator in the opportunistic PLL operates at the free-runningfrequency for the oscillator and in a closed loop condition wherein thereference frequency f_(REF) is approximately the free-running frequencyand the reference signal is synchronized with the clock signal.

Example 4 includes the subject matter of example 1, including oromitting optional elements, further including measurement circuitryconfigured to measure the frequency of the reference signal generated bythe opportunistic PLL to determine a measured reference frequencyf_(REF).

Example 5 includes the subject matter of example 4, including oromitting optional elements, further including control circuitryconfigured to determine a ratio (N) between the measured referencefrequency f_(REF) and the desired RF frequency f_(RF); adjust thereference frequency f_(REF) based on the ratio; and control a digitallycontrolled oscillator (DCO) in the RF PLL based on a phase differencebetween the reference signal and a feedback RF signal to synchronize theRF signal with the reference signal.

Example 6 includes the subject matter of example 5, including oromitting optional elements, wherein the control circuitry is configuredto determine the desired RF frequency f_(RF); determine an integerportion (A) of the ratio based on a quotient of the desired RF frequencyf_(RF) and the reference frequency f_(REF); determine a frequency moduloΔf_(RF) based on a difference between the desired RF frequency f_(RF)and a product of the integer portion A and the reference frequencyf_(REF); select a sub-integer ratio denominator (B) based on thefrequency modulo; determine a sub-integer ratio numerator n based on aproduct of the sub-integer ratio denominator B and the frequency moduloΔf_(RF) divided by the reference frequency f_(REF); set the ratio (N) toa sum of the integer portion A and a quotient of the sub-integer rationumerator n divided by the sub-integer ratio denominator B; and controlthe opportunistic PLL to generate the reference signal having thereference frequency f_(REF)=equal to a quotient of the desired RFfrequency f_(RF) divided by the ratio N.

Example 7 includes the subject matter of example 6, including oromitting optional elements, wherein the control circuitry is configuredto determine at least two ratios and select one of the at least tworatios that is a non-integer.

Example 8 includes the subject matter of example 5, including oromitting optional elements, wherein the control circuitry is furtherconfigured to determine that a re-calibration criterion is met and whenthe re-calibration criterion is met control the opportunistic PLL togenerate a new reference signal an open loop condition wherein anoscillator in the opportunistic PLL operates at a new free-runningfrequency for the oscillator and in a closed loop condition wherein anew reference frequency f_(REF) is approximately the new free-runningfrequency and the new reference signal is synchronized with the clocksignal. The control circuitry is configured to control the RF PLL todetermine a new ratio (N) between a new measured reference frequencyf_(REF) and the desired RF frequency f_(RF); adjust the new referencefrequency f_(REF) based on the ratio; and control the DCO based on aphase difference between the new reference signal and the feedback RFsignal to synchronize the RF signal with the new reference signal.

Example 9 is a method to generate a radio frequency (RF) signal having adesired RF frequency f_(RF), including with a clock circuitry,generating a clock signal having a frequency f_(XTL); with anopportunistic phase locked loop (PLL), generating a reference signalhaving a reference frequency f_(REF) that is close to a free-runningfrequency of an oscillator in the opportunistic PLL and synchronizingthe reference signal with the clock signal; with an RF PLL, generatingthe RF signal having the desired RF frequency f_(RF); and synchronizingthe RF signal with the reference signal.

Example 10 includes the subject matter of example 9, including oromitting optional elements, further including operating theopportunistic PLL in an open loop condition wherein an oscillator in theopportunistic PLL operates at the free-running frequency for theoscillator and operating the opportunistic PLL in a closed loopcondition wherein the reference frequency f_(REF) is approximately thefree-running frequency and the reference signal is synchronized with theclock signal.

Example 11 includes the subject matter of example 9, including oromitting optional elements, further including measuring the frequency ofthe reference signal generated by the opportunistic PLL to determine ameasured reference frequency f_(REF).

Example 12 includes the subject matter of example 11, including oromitting optional elements, further including determining a ratio (N)between the measured reference frequency f_(REF) and the desired RFfrequency f_(RF); adjusting the reference frequency f_(REF) based on theratio; and controlling a digitally controlled oscillator (DCO) in the RFPLL based on a phase difference between the reference signal and afeedback RF signal to synchronize the RF signal with the referencesignal.

Example 13 includes the subject matter of example 12, including oromitting optional elements, further including determining the desired RFfrequency f_(RF); determining an integer portion (A) of the ratio basedon a quotient of the desired RF frequency f_(RF) and the referencefrequency f_(REF); determining a frequency modulo Δf_(RF) based on adifference between the desired RF frequency f_(RF) and a product of theinteger portion A and the reference frequency f_(REF); selecting asub-integer ratio denominator (B) based on the frequency modulo;determining a sub-integer ratio numerator n based on a product of thesub-integer ratio denominator B and the frequency modulo Δf_(RF) dividedby the reference frequency f_(REF); setting the ratio (N) to a sum ofthe integer portion A and a quotient of the sub-integer ratio numeratorn divided by the sub-integer ratio denominator B; and controlling theopportunistic PLL to generate the reference signal having the referencefrequency f_(REF)=equal to a quotient of the desired RF frequency f_(RF)divided by the ratio N.

Example 14 includes the subject matter of example 12, including oromitting optional elements, further including determining at least tworatios and selecting one of the at least two ratios that is anon-integer as the ratio used by the RF PLL.

Example 15 includes the subject matter of example 12, including oromitting optional elements, further including determining that are-calibration criterion is met and when the re-calibration criterion ismet operating the opportunistic PLL in an open loop condition wherein anoscillator in the opportunistic PLL operates at a new free-runningfrequency for the oscillator and operating the opportunistic PLL in aclosed loop condition wherein a new reference frequency f_(REF) isapproximately the new free-running frequency and a new reference signalis synchronized with the clock signal. The method includes determining anew ratio (N) between a new measured reference frequency f_(REF) and thedesired RF frequency f_(RF); adjusting the new the reference frequencyf_(REF) based on the ratio; and controlling the DCO based on a phasedifference between the new reference signal and the feedback RF signalto synchronize the RF signal with the new reference signal.

Example 16 is control circuitry including hardware configured todetermine a desired radio frequency (RF) frequency f_(RF); measure areference frequency f_(REF) of a reference signal generated by anopportunistic phase locked loop (PLL); determine a ratio for an RF PLLthat generates an RF signal based on the desired RF frequency f_(RF) andthe reference frequency f_(REF); and determine an adjusted value of thereference frequency f_(REF) based on the ratio.

Example 17 includes the subject matter of example 16, including oromitting optional elements, further including hardware configured tocontrol the opportunistic PLL to generate the reference signal in anopen loop condition wherein an oscillator in the opportunistic PLLoperates at a free-running frequency for the oscillator and in a closedloop condition wherein the reference frequency f_(REF) is approximatelythe free-running frequency and the reference signal is synchronized witha clock signal.

Example 18 includes the subject matter of example 17, including oromitting optional elements, further including hardware configured todetermine that a re-calibration criterion is met and when there-calibration criterion is met control the opportunistic PLL togenerate a new reference signal in an open loop condition wherein anoscillator in the opportunistic PLL operates at a new free-runningfrequency for the oscillator and in a closed loop condition wherein anew reference frequency f_(REF) is approximately the new free-runningfrequency and the new reference signal is synchronized with a clocksignal. The control circuitry includes hardware configured to controlthe RF PLL to determine a new ratio (N) between a new measured referencefrequency f_(REF) and the desired RF frequency f_(RF); adjust the newreference frequency f_(REF) based on the ratio; and control a digitallycontrolled oscillator (DCO) in the RF PLL based on a phase differencebetween the new reference signal and a feedback RF signal to synchronizethe RF signal with the new reference signal.

Example 19 includes the subject matter of example 16, including oromitting optional elements, further including hardware configured todetermine an integer portion (A) of the ratio based on a quotient of thedesired RF frequency f_(RF) and the reference frequency f_(REF);determine a frequency modulo Δf_(RF) based on a difference between thedesired RF frequency f_(RF) and a product of the integer portion A andthe reference frequency f_(REF); select a sub-integer ratio denominator(B) based on the frequency modulo; determine a sub-integer rationumerator n based on a product of the sub-integer ratio denominator Band the frequency modulo Δf_(RF) divided by the reference frequencyf_(REF); set the ratio (N) to a sum of the integer portion A and aquotient of the sub-integer ratio numerator n divided by the sub-integerratio denominator B; and control the opportunistic PLL to generate thereference signal having the reference frequency f_(REF)=equal to aquotient of the desired RF frequency f_(RF) divided by the ratio N.

Example 20 includes the subject matter of example 16, including oromitting optional elements, further including hardware configured todetermine at least two ratios and select one of the at least two ratiosthat is a non-integer as the ratio used by the RF PLL.

Various illustrative logics, logical blocks, modules, and circuitsdescribed in connection with aspects disclosed herein can be implementedor performed with a general purpose processor, a digital signalprocessor (DSP), an application specific integrated circuit (ASIC), afield programmable gate array (FPGA) or other programmable logic device,discrete gate or transistor logic, discrete hardware components, or anycombination thereof designed to perform functions described herein. Ageneral-purpose processor can be a microprocessor, but, in thealternative, processor can be any conventional processor, controller,microcontroller, or state machine. The various illustrative logics,logical blocks, modules, and circuits described in connection withaspects disclosed herein can be implemented or performed with a generalpurpose processor executing instructions stored in computer readablemedium.

The above description of illustrated embodiments of the subjectdisclosure, including what is described in the Abstract, is not intendedto be exhaustive or to limit the disclosed embodiments to the preciseforms disclosed. While specific embodiments and examples are describedherein for illustrative purposes, various modifications are possiblethat are considered within the scope of such embodiments and examples,as those skilled in the relevant art can recognize.

In this regard, while the disclosed subject matter has been described inconnection with various embodiments and corresponding Figures, whereapplicable, it is to be understood that other similar embodiments can beused or modifications and additions can be made to the describedembodiments for performing the same, similar, alternative, or substitutefunction of the disclosed subject matter without deviating therefrom.Therefore, the disclosed subject matter should not be limited to anysingle embodiment described herein, but rather should be construed inbreadth and scope in accordance with the appended claims below.

In particular regard to the various functions performed by the abovedescribed components (assemblies, devices, circuits, systems, etc.), theterms (including a reference to a “means”) used to describe suchcomponents are intended to correspond, unless otherwise indicated, toany component or structure which performs the specified function of thedescribed component (e.g., that is functionally equivalent), even thoughnot structurally equivalent to the disclosed structure which performsthe function in the herein illustrated exemplary implementations of thedisclosure. In addition, while a particular feature may have beendisclosed with respect to only one of several implementations, suchfeature may be combined with one or more other features of the otherimplementations as may be desired and advantageous for any given orparticular application. The use of the phrase “one or more of A, B, orC” is intended to include all combinations of A, B, and C, for exampleA, A and B, A and B and C, B, and so on.

What is claimed is:
 1. A frequency synthesizer system configured to generate a radio frequency (RF) signal having a desired radio frequency f_(RF), comprising: a clock circuitry configured to generate a clock signal having a frequency f_(XTL); an opportunistic phase locked loop (PLL) configured to generate a reference signal having a reference frequency f_(REF) that is close to a free-running frequency of an oscillator in the opportunistic PLL, wherein the opportunistic PLL is configured to synchronize the reference signal to the clock signal; and an RF PLL configured to generate the RF signal having the desired radio RF frequency f_(RF) and to synchronize the RF signal with the reference signal.
 2. The frequency synthesizer system of claim 1, wherein the reference frequency f_(REF) is at least one order of magnitude greater than f_(XTL).
 3. The frequency synthesizer system of claim 1, wherein the opportunistic PLL is configured to generate the reference signal: in an open loop condition wherein the oscillator in the opportunistic PLL operates at the free-running frequency for the oscillator; and in a closed loop condition wherein the reference frequency f_(REF) is approximately the free-running frequency and the reference signal is synchronized with the clock signal.
 4. The frequency synthesizer system of claim 1, further comprising measurement circuitry configured to measure the frequency of the reference signal generated by the opportunistic PLL to determine a measured reference frequency f_(REF).
 5. The frequency synthesizer system of claim 4, further comprising control circuitry configured to: determine a ratio (N) between the measured reference frequency f_(REF) and the desired RF frequency f_(RF); adjust the reference frequency f_(REF) based on the ratio; and control a digitally controlled oscillator (DCO) in the RF PLL based on a phase difference between the reference signal and a feedback RF signal to synchronize the RF signal with the reference signal.
 6. The frequency synthesizer system of claim 5, wherein the control circuitry is configured to: determine the desired RF frequency f_(RF); determine an integer portion (A) of the ratio based on a quotient of the desired RF frequency f_(RF) and the reference frequency f_(REF); determine a frequency modulo Δf_(RF) based on a difference between the desired RF frequency f_(RF) and a product of the integer portion A and the reference frequency f_(REF); select a sub-integer ratio denominator (B) based on the frequency modulo; determine a sub-integer ratio numerator n based on a product of the sub-integer ratio denominator B and the frequency modulo Δf_(RF) divided by the reference frequency f_(REF); set the ratio (N) to a sum of the integer portion A and a quotient of the sub-integer ratio numerator n divided by the sub-integer ratio denominator B; and control the opportunistic PLL to generate the reference signal having the reference frequency f_(REF)=equal to a quotient of the desired RF frequency f_(RF) divided by the ratio N.
 7. The frequency synthesizer system of claim 6, wherein the control circuitry is configured to: determine at least two ratios; and select one of the at least two ratios that is a non-integer.
 8. The frequency synthesizer system of claim 5, wherein the control circuitry is further configured to: determine that a re-calibration criterion is met; when the re-calibration criterion is met: control the opportunistic PLL to generate a new reference signal: an open loop condition wherein an oscillator in the opportunistic PLL operates at a new free-running frequency for the oscillator; and in a closed loop condition wherein a new reference frequency f_(REF) is approximately the new free-running frequency and the new reference signal is synchronized with the clock signal; and control the RF PLL to: determine a new ratio (N) between a new measured reference frequency f_(REF) and the desired RF frequency f_(RF); adjust the new reference frequency f_(REF) based on the ratio; and control the DCO based on a phase difference between the new reference signal and the feedback RF signal to synchronize the RF signal with the new reference signal.
 9. A method to generate a radio frequency (RF) signal having a desired RF frequency f_(RF), comprising: with a clock circuitry, generating a clock signal having a frequency f_(XTL); with an opportunistic phase locked loop (PLL): generating a reference signal having a reference frequency f_(REF) that is close to a free-running frequency of an oscillator in the opportunistic PLL; and synchronizing the reference signal with the clock signal; with an RF PLL: generating the RF signal having the desired RF frequency f_(RF); and synchronizing the RF signal with the reference signal.
 10. The method of claim 9, further comprising: operating the opportunistic PLL in an open loop condition wherein an oscillator in the opportunistic PLL operates at the free-running frequency for the oscillator; and operating the opportunistic PLL in a closed loop condition wherein the reference frequency f_(REF) is approximately the free-running frequency and the reference signal is synchronized with the clock signal.
 11. The method of claim 9, further comprising measuring the frequency of the reference signal generated by the opportunistic PLL to determine a measured reference frequency f_(REF).
 12. The method of claim 11, further comprising: determining a ratio (N) between the measured reference frequency f_(REF) and the desired RF frequency f_(RF); adjusting the reference frequency f_(REF) based on the ratio; and controlling a digitally controlled oscillator (DCO) in the RF PLL based on a phase difference between the reference signal and a feedback RF signal to synchronize the RF signal with the reference signal.
 13. The method of claim 12, further comprising: determining the desired RF frequency f_(RF); determining an integer portion (A) of the ratio based on a quotient of the desired RF frequency f_(RF) and the reference frequency f_(REF); determining a frequency modulo Δf_(RF) based on a difference between the desired RF frequency f_(RF) and a product of the integer portion A and the reference frequency f_(REF); selecting a sub-integer ratio denominator (B) based on the frequency modulo; determining a sub-integer ratio numerator n based on a product of the sub-integer ratio denominator B and the frequency modulo Δf_(RF) divided by the reference frequency f_(REF); setting the ratio (N) to a sum of the integer portion A and a quotient of the sub-integer ratio numerator n divided by the sub-integer ratio denominator B; and controlling the opportunistic PLL to generate the reference signal having the reference frequency f_(REF)=equal to a quotient of the desired RF frequency f_(RF) divided by the ratio N.
 14. The method of claim 12, further comprising: determining at least two ratios; and selecting one of the at least two ratios that is a non-integer as the ratio used by the RF PLL.
 15. The method of claim 12, further comprising: determining that a re-calibration criterion is met; when the re-calibration criterion is met: operating the opportunistic PLL in an open loop condition wherein an oscillator in the opportunistic PLL operates at a new free-running frequency for the oscillator; and operating the opportunistic PLL in a closed loop condition wherein a new reference frequency f_(REF) is approximately the new free-running frequency and a new reference signal is synchronized with the clock signal; determining a new ratio (N) between a new measured reference frequency f_(REF) and the desired RF frequency f_(RF); adjusting the new the reference frequency f_(REF) based on the ratio; and controlling the DCO based on a phase difference between the new reference signal and the feedback RF signal to synchronize the RF signal with the new reference signal.
 16. Control circuitry, comprising hardware configured to: determine a desired radio frequency (RF) frequency f_(RF); measure a reference frequency f_(REF) of a reference signal generated by an opportunistic phase locked loop (PLL); determine a ratio for an RF PLL that generates an RF signal based on the desired RF frequency f_(RF) and the reference frequency f_(REF); and determine an adjusted value of the reference frequency f_(REF) based on the ratio.
 17. The control circuitry of claim 16, further comprising hardware configured to control the opportunistic PLL to generate the reference signal: in an open loop condition wherein an oscillator in the opportunistic PLL operates at a free-running frequency for the oscillator; and in a closed loop condition wherein the reference frequency f_(REF) is approximately the free-running frequency and the reference signal is synchronized with a clock signal.
 18. The control circuitry of claim 17, further comprising hardware configured to: determine that a re-calibration criterion is met; when the re-calibration criterion is met: control the opportunistic PLL to generate a new reference signal: in an open loop condition wherein an oscillator in the opportunistic PLL operates at a new free-running frequency for the oscillator; and in a closed loop condition wherein a new reference frequency f_(REF) is approximately the new free-running frequency and the new reference signal is synchronized with a clock signal; and control the RF PLL to: determine a new ratio (N) between a new measured reference frequency f_(REF) and the desired RF frequency f_(RF); adjust the new reference frequency f_(REF) based on the ratio; and control a digitally controlled oscillator (DCO) in the RF PLL based on a phase difference between the new reference signal and a feedback RF signal to synchronize the RF signal with the new reference signal.
 19. The control circuitry of claim 16, further comprising hardware configured to: determine an integer portion (A) of the ratio based on a quotient of the desired RF frequency f_(RF) and the reference frequency f_(REF); determine a frequency modulo Δf_(RF) based on a difference between the desired RF frequency f_(RF) and a product of the integer portion A and the reference frequency f_(REF); select a sub-integer ratio denominator (B) based on the frequency modulo; determine a sub-integer ratio numerator n based on a product of the sub-integer ratio denominator B and the frequency modulo Δf_(RF) divided by the reference frequency f_(REF); set the ratio (N) to a sum of the integer portion A and a quotient of the sub-integer ratio numerator n divided by the sub-integer ratio denominator B; and control the opportunistic PLL to generate the reference signal having the reference frequency f_(REF)=equal to a quotient of the desired RF frequency f_(RF) divided by the ratio N.
 20. The control circuitry of claim 16, further comprising hardware configured to: determine at least two ratios; and select one of the at least two ratios that is a non-integer as the ratio used by the RF PLL. 